Suppose I'm running a webserver on a slow dialup link, and I'm serving large pages containing mostly static content with occasional dynamic sections. It's therefore possible that a site with a better connection that knows how to generate the static bits of my outgoing data can generate packets that appear to come from me with the appropriate contents, exchanging some minimal information with my machine (TCP sequencing, etc.). Effectively, this is distributing one end of a TCP connection across several hosts.
Werner Almesberger's already done essentially this as TCP connection passing.